Robotics, prosthetic devices and other assistive devices use high reduction ratio gear trains, with gear ratios typically in the range of 10:1 to 1000:1 to convert relatively high motor speeds at relatively low torque, to lower speeds at relatively higher torque. This may be accomplished using harmonic drives, cycloid drives, planetary drives, or worm drives. However, these gear trains are often relatively heavy and expensive to manufacture. In some instances, the gear train of a single joint of a human scale robot weighs as much as 5 pounds and cost hundreds, if not thousands, of dollars. Such heavy-weight gear trains may make robot arms and assistive devices unsafe when operating at high speeds around humans due to their kinetic energy. Robots must often be kept in safety cages or behind fences, separated from people. In the case of prosthetic and assistive devices, the added expense limits access to the technology and the weight limits design flexibility.
Accordingly, there remains the need to develop gear trains that are relatively inexpensive to produce and are torque-limited to enable humans and robots to work together. Weight reduction is also desirable to increase robot payload, or to reduce energy consumption and user fatigue when incorporated into prosthetic and assistive devices.